Doped refractory with a high zirconia content

ABSTRACT

The invention provides a fused cast refractory product with a high zirconia content comprising, as. a percentage by weight based on the oxides:
         ZrO 2 +Hf 2 O: &gt;85%;   SiO 2 : 6% to 12%;   Al 2 O 3 : 0.4% to 1%;   Y 2 O 3 : 0.2%;   a dopant selected from the group formed by Nb 2 O 5 , Ta 2 O 5 , and mixtures thereof, in a quantity such that the molar ratio ZrO 2 /(Nb 2 O 5 +Ta 2 O 3 ) is in the range 200 to 350.

The invention relates to a novel fused cast refractory product with a high zirconia content.

Refractory products include fused cast products, which are well known in the construction of glass-melting furnaces, and sintered products.

In contrast to sintered products such as those described, for example, in U.S. Pat. No. 4,507,394, fused cast products usually include an intergranular vitreous phase connecting together the crystalline grains. The problems posed by sintered products and by fused cast products, and the technical solutions adopted to overcome them thus generally differ. A composition that has been developed for the production of a sintered product is thus not, a priori, of use per se in producing a fused cast product, and vice versa.

Fused cast products, often termed electrofused products, are obtained by melting a mixture of appropriate starting materials in an electric arc furnace or by using any other technique that is suitable for such products. The molten liquid is then cast into a mold and the product obtained undergoes a controlled cooling cycle to bring it to ambient temperature without fracturing. This operation is termed “annealing” in the art.

Fused cast products include electrofused products with a high zirconia content, i.e. comprising more than 85% by weight of zirconia ZrO₂); (they are well known for their very good resistance to corrosion without coloring the glass produced and without generating defects.

Conventionally, fused cast products with a high zirconia content also include sodium oxide (Na₂O) to prevent the formation of zircon from the zirconia and silica present in the product. Zircon formation is in fact deleterious, since it is accompanied by a reduction in volume of the order of 20%, thus creating mechanical stresses that are the source of cracks.

The product ER-1195, produced and marketed by the Société Européenne des Produits Réfractaires and protected by European patent EP-B-0 403 387, is currently widely used in glass-melting furnaces. Its chemical composition comprises approximately 94% of zirconia, 4% to 5% of silica, approximately 1% of alumina, 0.3% of sodium oxide and less than 0.05% by weight of P₂O₅. It is typical of products with a high zirconia content that are used for glass furnaces.

French patent FR-A-2 701 022 describes fused cast products with a high zirconia content that contain 0.05% to 1.0% by weight of P₂O₅ and 0.05% to 1.0% by weight of boron oxide, B₂O₃. Said products have a high electrical resistivity. This can advantageously stabilize the consumption of electricity during electric melting of the glass and above all, this can overcome any problems with short circuiting in refractories, causing their rapid degradation. During electric melting of glass, some of the electric current passes through the refractory products. Thus, increasing the resistivity of said refractory products can reduce the quantity of electric current that passes through them.

International patent document WO-A-2005/068393 describes fused cast products with a high zirconia content having a high electrical resistivity while minimizing the quantities of BaO, SrO, MgO, CaO, P₂O₅, Na₂O, and K₂O. Said products contain 0.1% to 1.2% by weight of B₂O₃.

Japanese patent document JP 2000 302 560 describes fused cast products that do not contain Nb₂O₅ or Ta₂O₅.

The current trend for very high quality glass, in particular glass for LCD type flat screens, is increasing the demand for refractory products from glass-melting furnaces. In particular, there is a need for refractory products with further improved electrical resistivity while retaining good feasibility and good service characteristics.

The aim of the present invention is to satisfy this need.

More particularly, in a first aspect, the invention provides a fused cast refractory product with a high zirconia content comprising, as a percentage by weight based on the oxides;

-   -   ZrO₂+Hf₂O: >85%;     -   SiO₂: 6% to 12%;     -   Al₂O₃: 0.4% to 1%;     -   Y₂O₃: ≦0.2%; and     -   a dopant selected from the group formed by Nb₂O₅, Ta₂O₅ and         mixtures thereof.

As can be seen below, surprisingly, the inventors have discovered that this composition enables a refractory product in accordance with the invention to have remarkable electrical resistivity while retaining good feasibility and good service characteristics.

The refractory product of the invention may also have one or more of the following optional characteristics:

-   -   the ZrO₂/(Nb₂O₅+Ta₂O₅) molar ratio is more than 200 and/or leas         than 350;     -   the total quantity of dopant is 0.2% or more, preferably 0.25%         or more and/or 5% or less, 3% or less, 1% or less, 0.8% or less,         0.5% or less, preferably 0.4% or less, as a molar percentage         based on the oxides;     -   the quantity by weight of dopant Ta₂O₅ is 0.1% or more,         preferably 0.2% or more, and/or 2.5% or less, preferably 1.5% or         less;     -   the quantity by weight of dopant Nb₂O₅ is 0.1% or more,         preferably 0.2% or more, and/or 1.5% or less, preferably 1.0% or         less;     -   at least 80%, preferably at least 90%, or even at least 99% or         substantially 100% of the zirconia is unstabilized monoclinic,         the percentages being by weight;     -   the quantity by weight of silica, SiO₂, is 6.5% or more,         preferably 7% or more. It may be 10% or less, or even 9% or         less. In a variation, it may also be more than 8.5%, or even 9%,         or even 10%, or even more than 10.1%, or even more than 10.5%;     -   the quantity by weight of boron oxide, B₂O₃, is 0.2% or more,         preferably 0.3% or more and/or 1.0% or less, preferably 0.8% or         less;     -   the quantity by weight of sodium oxide, Na₂O, is 0.1% or less,         preferably 0.05% or less, more preferably 0.03% or less;     -   the quantity by weight of alumina, Al₂O₃, is preferably 0.5% or         more and/or preferably 0.8% or less;     -   the species other than ZrO₂+HfO₂, SiO₂, B₂O₃, Al₂O₃, Y₂O₃, BaO,         Ta₂O₅, and Nb₂O₅ represent, by weight, less than 1.5%,         preferably less than 1%, more preferably less than 0.5%;     -   the species other than ZrO₂+HfO₂, SiO₂, B₂O₃, Al₂O₃, Y₂O₃,         Ta₂O₅, and Nb₂O₅ represent, by weight, less than 1.5%,         preferably less than 1%, more preferably less than 0.5%;     -   the quantity by weight of barium oxide, BaO, is 0.6% or less,         preferably less than 0.5%;     -   the complement by weight to 100% is constituted by impurities,         the total impurities content preferably being less than 0.5%;         and     -   the refractory product has the following composition by weight,         with ZrO₂ HfO₂ and the impurities as the complement to 100%:         -   SiO₂: 7% to 9%;         -   B₂O₃: 0.3% to 0.8%;         -   Al₂O₃: 0.4% to 0.8%;         -   Y₂O₃: ≦0.2%; and         -   in the range 0.2% to 1.5% of Ta₂O₅ and/or in the range 0.2%             to 1.0% of Nb₂O₅.

Advantageously, these characteristics can further improve the electrical resistivity and the corrosion resistance of the product of the invention.

The refractory product of the invention has an electrical resistivity of 500 Ω·cm [ohm·centimeter] or more, preferably 600 Ω·cm or more, at 1500° C. at a frequency of 100 Hz [hertz].

In accordance with a further aspect, the invention provides a fused cast refractory product with a high zirconia content comprising, as a percentage by weight based on the oxides:

-   -   ZrO₂+Hf₂O: >85%;     -   SiO₂: 6% to 12%;     -   Y₂O₃: 0.2%; and     -   a dopant selected from Ta₂O₅ and Nb₂O₅, and preferably from         mixtures of Nb₂O₅ and Ta₂O₅, in which Nb₂O₅>0.1% and, with         mixtures of Nb₂O₅ and Ta₂O₅, Ta₂O₅>0.1%, as a percentage by         weight based on the oxides and when Ta₂O₅ is the only dopant,         Ta₂O₅≦1.5%.

The refractory product of the invention may also have one more of the following optional characteristics:

-   -   the ZrO₂/(Nb₂O₅+Ta₂O₅) molar ratio is in the range 200 to 350;     -   the total quantity of dopant is 0.2% or more, preferably 0.25%         or more and/or 5% or less, 3% or less, 1% or less, 0.8% or less,         0.5% or less, preferably 0.4% or less, as a molar percentage         based on the oxides;     -   the quantity by weight of dopant Ta₂O₅ is 0.2% or more and/or         2.5% or less, preferably 1.5% or less;     -   the quantity by weight of dopant Nb₂O₅ is 0.2% or more and/or         1.5% or less, preferably 1.0% or less;     -   at least 80%, preferably at least 90%, or even at least 99% or         substantially 100% of the zirconia is monoclinic, the         percentages being by weight;     -   the quantity by weight of silica, SiO₂, is 6.5% or more,         preferably 7% or more. It may be 10% or less, or even 9% or         less. In a variation, it may also be more than 8.5%, or even 9%,         or even 10%, or even more than 10.1%, or even more than 10.5%;     -   the quantity by weight of boron oxide, B₂O₃, is 0.2% or more,         preferably 0.3% or more and/or 1.0% or less, preferably 0.8% or         less;     -   the quantity by weight of sodium oxide, Na₂O, is 0.1% or less,         preferably 0.05% or less, more preferably 0.03% or less;     -   the quantity by weight of alumina, Al₂O₃, is preferably 0.4% or         more, preferably 0.5% and/or preferably 1% or less, preferably         0.8%;     -   the quantity by weight of barium oxide, BaO, is 0.6% or less,         preferably less than 0.5%;     -   the species other than ZrO₂HfO₂, SiO₂, B₂O₃, Al₂O₃, Y₂O₃, BaO,         Ta₂O₅, and Nb₂O₅ represent, by weight, less than 1.5%,         preferably less than 1%, more preferably less than 0.5%; and     -   the species other than ZrO₂+HfO₂, SiO₂, B₂O₃, Al₂O₃, Y₂O₃,         Ta₂O₅, and Nb₂O₅ represent, by weight, less than 1.5%,         preferably less than 1%, more preferably less than 0.5%;     -   the complement to 100% by weight is constituted by impurities,         the total impurities content preferably being less than 0.5%.

In accordance with a further aspect, the invention Provides a fused cast refractory product with a high zirconia content comprising:

-   -   ZrO₂+Hf₂O: >85%;     -   SiO₂: 6% to 12%;     -   Y₂O₃: ≦0.2%;     -   a dopant selected from mixtures of Nb₂O₅ and Ta₂O₅, in which         Ta₂O₅<1%, as a percentage by weight based on the oxides.

Preferably, the ZrO₂/(Nb₂O₅+Ta₂O₅) molar ratio is more than 200 and less than 350.

The refractory product of the invention may also have one more of the following optional characteristics:

-   -   the total quantity of dopant is 0.2% or more, preferably 0.25%         or more and/or 5% or less, 3% or less, 1% or less, 0.8% or less,         0.5% or less, preferably 0.4% or less, as a molar percentage         based on the oxides;     -   the quantity by weight of dopant Nb₂O₅ is 0.1% or more,         preferably 0.2% or more and/or 1.5% or less, preferably 1.0% or         less;     -   at least 80%, preferably at least 90%, or even at least 99% or         substantially 100% of the zirconia is unstabilized monoclinic,         the percentages being by weight;     -   the quantity by weight of silica, SiO₂, is 6.5% or more,         preferably 7% or more. It may be 10% or less, or even 9% or         less. In a variation, it may also be more than 8.5%, or even 9%,         or even 10%, or even more than 10.1%, or even more than 10.5%;     -   the quantity by weight of boron oxide, B₂O₃, 0.2% or more,         preferably 0.3% or more and/or 1.0% or less, preferably 0.8% or         less;     -   the quantity by weight of sodium oxide, Na₂O, is 0.1% or less,         preferably 0.05% or less, more preferably 0.03% or less;     -   the quantity by weight of alumina, Al₂O₃, is preferably 0.4% or         more, preferably 0.5% and/or preferably 1% or less, preferably         0.8%;     -   the quantity by weight of barium oxide, BaO, is 0.6% or less,         preferably less than 0.5%;     -   the species other than ZrO₂+HfO₂, SiO₂, B₂O₃, Al₂O₃, Y₂O₃, BaO,         Ta₂O₅ and Nb₂O₅ represent, by weight, less than 1.5%, preferably         less than 1%, more preferably less than 0.5%;     -   the species other than ZrO₂+HfO₂, SiO₂, B₂O₃, Al₂O₃, Y₂O₃, Ta₂O₅         and Nb₂O₅ represent, by weight, less than 1.5%, preferably less         than 1%, more preferably less than 0.5%; and     -   the complement to 100% by weight is constituted by impurities,         the total impurities content preferably being less than 0.5%.

The invention also provides a glass-melting furnace including a refractory product in accordance with the invention, in particular in regions intended to come into contact with molten glass.

Finally, the invention provides a method of producing a refractory product in accordance with the invention, comprising the following steps in succession:

a) mixing starting materials, with the introduction of a dopant, to form a starting charge;

b) melting said starting charge until a molten liquid is obtained;

c) casting and solidifying said molten liquid by controlled cooling to obtain a refractory product;

said method being remarkable in that said starting materials are selected such that said refractory product is in accordance with the invention.

Unless otherwise mentioned, all of the percentages in the present description are percentages by weight based on the oxides.

In the fused cast products of the invention, the high zirconia content, i.e. ZrO₂>85%, means that it can satisfy the demands for high corrosion resistance without coloring the glass produced and without generating defects that are deleterious to the quality of said glass.

Hafnium oxide, HfO₂, present in the product of the invention is the hafnium oxide naturally present in sources of zirconia. Its quantity by weight in the product of the invention is thus 5% or less, generally 2% or less.

The presence of silica is necessary for the formation of an intergranular vitreous phase that can effectively accommodate variations in the volume of the zirconia during its reversible allotropic transformation, i.e. during passage from the monoclinic phase to the tetragonal phase.

The inventors have discovered that a quantity by weight of silica of more than 6% can produce higher electrical resistivities.

In contrast, the added silica must not exceed 12% by weight since, by reducing the zirconia content, it would cause the corrosion resistance would be too greatly reduced. Further, too high a quantity of silica could generate defects in the glass by releasing stones (Pieces of refractory product resulting from a loss of cohesion of the product), which is considered to be a poor service characteristic.

The presence of alumina is favourable for the formation of a stable vitreous phase and for good castability of the products into the mold. An excessive amount causes instability of the vitreous phase (crystal formation), which has a negative impact on feasibility. Preferably, the content by weight of alumina must thus remain below 1.0%.

Preferably, the product of the invention comprises a quantity by weight of B₂O₃ of more than 0.3%, preferably more than 0.4%. The feasibility is improved thereby. The B₂O₃ content must preferably remain 1.0% or less. Advantageously, the formation of zircon in the product is limited.

Yttrium oxide, Y₂O₃, has an unfavorable effect on electrical resistivity. Its quantity by weight must thus be less than 0.2%.

The presence of dopant in the products of the invention is necessary in order to improve the electrical resistivity. However, the total content of said dopant oxides must not be too high so that the percentage of zirconia is kept at a sufficiently high level to ensure good resistance to corrosion by the molten glass and to retain good stability in the vitreous phase.

The inventors have established that the dopants Nb₂O₅ and Ta₂O₅ have a substantially identical effect at identical molar quantities. Without wishing to be bound by any particular theory, the inventors explain this effect by the role played by the dopants as regards oxygen voids in the zirconia. The dopants would in fact each compensate for one oxygen void.

Thus, 1.66 g [gram] of Ta₂O₅ has an equivalent effect to 1 g of Nb₂O₅.

The inventors have established that the electrical resistivity is particularly enhanced when the ratio between the molar percentage of zirconia and the molar percentage of dopant is in the range 200 to 350. This range corresponds to an optimum doping of zirconia grains. It indicates the quantity of dopant necessary and sufficient to fill the oxygen voids.

The complement by weight to 100% in the composition of the product of the invention is constituted by other species. Preferably, the “other species” are merely impurities.

The term “impurities” means inevitable constituents necessarily introduced with the starting materials or resulting from reactions with these constituents. Hafnium oxide present in the zirconia source is not considered to be an impurity. Impurities are constituents that are not necessary but that are merely accepted.

Examples that may be mentioned are alkaline oxides, in particular sodium oxide, Na₂O, and potassium oxide, K₂O, which can be accepted but preferably must not exceed 0.2% by weight, more preferably 0.1%, and more preferably be present in trace amounts only. Otherwise, the electrical resistivity would be degraded because of the increased conductivity of the vitreous phase. Oxides of iron, titanium, and phosphorus are known to be harmful, and their contents must be limited to the traces introduced with the starting materials as impurities. Preferably, the quantity by weight of Fe₂O₃+TiO₂ is less than 0.55% and that of P₂O₅ is less than 0.05%.

The product of the invention is constituted by grains of unstabilized monoclinic zirconia surrounded by a vitreous phase principally constituted by silica. Without wishing to be bound by one theory, the inventors consider that the composition of the product of the invention can maximize the electrical resistivity both of the grains of zirconia and the vitreous phase by obtaining a synergistic effect with all of the constituents.

A product of the invention may be produced by following steps a) to c) described below:

a) mixing starting materials, with the introduction of a dopant, to form a starting charge;

b) melting said starting charge until a molten liquid is obtained;

c) solidifying said molten liquid by controlled cooling to obtain a refractory product in accordance with the invention.

In step a), the dopant is added in a manner that guarantees the quantity of dopant in the finished product of the invention.

In step b), melting is preferably carried out by the combined action of a fairly long electric arc that does not cause reduction and stirring, favoring re-oxidation of the products. Melting is carried out at a temperature of more than 2300° C., preferably in the range 2400° C. to 2500° C.

To minimize the formation of nodules with a metallic appearance and to prevent the formation of apertures or crazing in the finished product, it is preferable to carry out melting under oxidizing conditions.

Preferably, a long arc fusion method is used, as described in French patent FR-A-1 208 577 and its patents of addition, numbers 75893 and 82310.

That method consists in using an electric arc furnace the arc of which arcs between the charge and at least one electrode that is separated from said charge, and adjusting the length of the arc so that its reducing action is reduced to a minimum, while maintaining an oxidizing atmosphere above the melt and stirring said melt either by the action of the arc itself or by bubbling an oxidizing gas (air or oxygen, for example) into the melt, or by adding to the melt substances which release oxygen, such as peroxides.

In step c), cooling is preferably carried out at a rate of approximately 10° C. per hour.

Any conventional method of producing fused products based on zirconia intended for applications in glass-melting furnaces may be employed, provided that the composition of the starting charge allows products to be produced with a composition that is in accordance with that of the product of the invention.

The following non-limiting examples are given with the aim of illustrating the invention.

In these examples, the following starting materials were used:

-   -   zirconia principally containing, as a mean percentage by weight,         98.5% of ZrO₂+HfO₂, 0.2% of SiO₂ and 0.02% of Na₂O;     -   zircon sand containing 33% silica;     -   type AC44 alumina sold by Pechiney and containing a mean of         99.4% of alumina Al₂O₃;     -   oxides of barium, boron, yttrium, tantalum, Ta₂O₅, and niobium,         Nb₂O₅, with a purity of more than 99%.

The examples were prepared using the conventional arc furnace melting method then cast to obtain blocks with a 220×450×150 mm [millimeter] format.

The chemical analysis of the products obtained is given in Table 1; it is a mean chemical analysis given as a percentage by weight with the exception of the column indicating the molar percentage of the sum of the oxides Ta₂O₅ and Nb₂O₅.

In this table, a blank entry corresponds to a quantity of 0.05% by weight or less.

Thus, the Na₂O content is not indicated, since it was always less than 0.05% by weight.

The ratio Z/D corresponds to the ratio of the molar percentage of zirconia over the molar percentage of the sum of the oxides Ta₂O₅ and Nb₂O₅.

Cylindrical bars of product 30 mm in diameter and 30 mm in height were removed from the various samples of the blocks produced. These bars were subjected to a potential difference of 1 volt at a frequency of 100 Hz at 1500° C. to carry out the measurements of electrical resistivity R.

TABLE 1 Ta₂O₅ + Nb₂O₅ R ZrO₂ SiO₂ B₂O₃ Al₂O₃ Nb₂O₅ Ta₂O₅ Y₂O₃ BaO (mol %) Z/D (Ω · cm) 1 91.3 6.1 0.65 0.93 0.38 0.48 0.20 0.29 294 675 2 91.3 6.1 0.60 0.95 0.40 0.49 0.19 0.30 283 620 3 91.4 6.1 0.65 0.90 0.37 0.30 0.20 0.24 358 347 4 91.3 6.7 0.60 0.50 0.51 0.20 0.24 0.22 386 434 5 91.3 7.0 0.60 1.10 95 6 90.3 7.1 0.60 0.79 0.33 0.76 0.20 0.34 247 507 7 91.1 7.5 0.60 0.44 0.17 0.13 253 8 90.0 8.1 0.41 0.54 0.74 0.20 0.32 262 780 9 89.8 8.3 0.60 1.20 0.10 165 10 89.2 8.3 0.49 0.80 0.31 0.70 0.20 0.31 263 918 11 89.8 8.4 0.48 0.44 0.66 0.20 0.28 294 746 12 89.3 8.9 0.49 0.47 0.71 0.20 0.30 271 857 13 87.6 9.0 0.65 0.60 0.37 0.61 0.19 1.10 0.31 256 861 14 88.6 9.3 0.54 0.50 0.76 0.20 0.13 0.32 252 899

These examples show that satisfying the criterion 200≦Z/D≦350 can produce electrical resistivities R of more than 500 Ω·cm, in particular for silica contents of less than 7.0%. For silica contents of more than 7.0%, such resistivities are advantageously possible even without satisfying said criterion.

Further, other trials have verified that the other recognized properties for high zirconia materials, in particular resistance to corrosion by glass, are not degraded by the presence of a dopant in accordance with the invention.

Clearly, the present invention is not limited to the implementations described and shown that are provided by way of illustrative, non-limiting example. 

1. A fused cast refractory product with a high zirconia content comprising, as a percentage by weight based on the oxides: ZrO₂+Hf₂O: >85%; SiO₂: 6% to 12%; Al₂O₃: 0.4% to 1%; Y₂O₃: ≦0.2%; a dopant selected from the group formed by Nb₂O₅, Ta₂O₅, and mixtures thereof.
 2. A refractory product according to claim 1, in which the ZrO₂/(Nb₂O₅+Ta₂O₅) molar ratio is in the range 200 to
 350. 3. A refractory product according to claim 1, in which the quantity by weight of sodium oxide, Na₂O, is 0.1% or less.
 4. A refractory product according to claim 1, in which the quantity by weight of barium oxide, BaO, is 0.6% or less.
 5. A refractory product according to claim 1, in which the quantity by weight of dopant Ta₂O₅ is 0.1% or more and/or the quantity by weight of dopant Nb₂O₅ is 0.1% or more.
 6. A refractory product according to claim 1, in which the quantity by weight of dopant Ta₂O₅ is 0.2% or more and/or the quantity by weight of dopant Nb₂O₅ is 0.2% or more.
 7. A refractory product according to claim 1, in which the quantity by weight of dopant Ta₂O₅ is 2.5% or less and/or the quantity by weight of dopant Nb₂O₅ is 1.5% or less.
 8. A refractory product according to claim 1, in which the quantity by weight of dopant Ta₂O₅ is 1.5% or less and/or the quantity by weight of dopant Nb₂O₅ is 1.0% or less.
 9. A refractory product according to claim 1, in which the quantity by weight of silica, SiO₂, is 6.5% or more and/or 10% or less.
 10. A refractory product according to claim 1, in which the quantity by weight of silica, SiO₂, is 7.0% or more and/or 9% or less.
 11. A refractory product according to claim 1, in which the quantity by weight of alumina, Al₂O₃, is 0.5% or more and 0.8% or less.
 12. A refractory product according to claim 1, further comprising B₂O₃, the quantity of B₂O₃ being 0.2% or more and 1.0% or less.
 13. A refractory product according to claim 12, in which the quantity by weight of B₂O₃ is 0.3% or more and 0.8% or less.
 14. A refractory product according to claim 1, in which the species other than ZrO₂+HfO₂, SiO₂, B₂O₃, Al₂O₃, Y₂O₃, BaO, Ta₂O₅, and Nb₂O₅ represent less than 1.5% by weight.
 15. A refractory product according to claim 1, in which the complement by weight to 100% is constituted by impurities.
 16. A refractory product according to claim 15, in which the weight content of the impurities is less than 0.5%.
 17. A refractory product according to claim 1, having the following composition by weight, with ZrO₂+HfO₂ and the impurities as the complement to 100%: SiO₂: 7% to 9%; B₂O₃: 0.4% to 0.8%; Al₂O₃: 0.5% to 0.8%; Y₂O₃: ≦0.2%; in the range of 0.2% to 1.5% of Ta₂O₅ and/or in the range of 0.2% to 1.0% of Nb₂O₅.
 18. A refractory product according to claim 1, in which the total quantity of dopant is 0.5% or less, as a molar percentage based on the oxides.
 19. A refractory product according to claim 1, in which the total quantity of dopant is 0.4% or less, as a molar percentage based on the oxides.
 20. A refractory product according to claim 1, in which the total quantity of dopant is 0.2% or more, as a molar percentage based on the oxides.
 21. A glass-melting furnace, comprising a refractory product in accordance with claim
 1. 